Polycarbonate is a thermoplastic that has excellent mechanical properties such as impact resistance, heat resistance and transparency. Polycarbonates are widely used in applications ranging from football helmets to automobile parts to transparent security windows. More recently, polycarbonates have also proven to be the material of choice for optical media applications such as optical discs, for example compact discs (CD) and digital versatile discs (DVD). Conventional polycarbonates are usually produced by (1) an interfacial polymerization, in which bisphenol A (BPA) is reacted directly with phosgene or (2) a melt polymerization process in which BPA is transesterified with a carbonic acid diester such as diphenyl carbonate (DPC). For many applications, there has been a need for materials possessing the fundamental characteristics of transparency and toughness inherent in BPA polycarbonate but possessing, in addition, certain improvements in physical properties relative those possessed by bisphenol A polycarbonate (BPA-PC), for example reduced birefringence for optical applications. For some applications improved chemical resistance relative to BPA polycarbonate is required, for example in certain medical and automotive applications. Copolycarbonates are materials frequently possessing the fundamental traits of BPA polycarbonate, transparency and toughness, but in certain instances also possessing improved performance characteristics for a given application relative to BPA polycarbonate.
One example of such a copolycarbonate comprises repeat units derived from resorcinol or hydroquinone in addition to repeat units derived from bisphenol A. The incorporation of resorcinol-derived and hydroquinone-derived repeat units into a BPA-polycarbonate confers excellent melt flow properties, molding properties, solvent and heat resistance, while maintaining the excellent mechanical properties and transparency inherent in bisphenol A polycarbonate. Such copolycarbonates can be prepared by interfacial polymerization, melt polymerization, or solid state polymerization. (U.S. Pat. No. 6,177,536). The present invention relates to an improved method to prepare these and related copolycarbonates using the melt polymerization method.